The present invention relates to electromechanical precision rotary drives.
Such electromagnetic precision rotary drives are needed, e.g., for measuring wheels which are equipped with optical components. The measuring wheels are often used in evacuated cryostats. The measuring wheel is driven by a rotor, which forms part of the electromagnetic rotary drive. The rotor may itself be the measuring wheel.
For such precision rotary drives operated in fixed uniform steps, stringent requirements are set in several respects.
Thus, the angle step width must be very accurately reproducible. Normal anticipated values are below 10 um, referred to the outside radius of the measuring wheel.
The measuring wheel and the optical components fastened thereon must not vibrate excessively either when the rotary drive starts up or when it stops. Even slight vibrations may lead to inaccuracies in the measurement results or to unnecessary delays.
If a precision drive is used, as described above, inside a cryostat, the number of electrical connections needed for the operation of the rotary drive must be minimized. Normally only two connecting wires, or in the case of a redundant drive three connecting wires, are available. Due to this limitation, precise multi-phase step drives, for example, cannot be employed.
Furthermore, in the precision rotary drive the power consumption per angle step executed should be low; thus, no closed-circuit power should be consumed in the set and maintained position of the measuring wheel.
Also, the rotary drive is to operate in vacuum as well as under air pressure in a wide temperature range between about 4K and 300K. It is to be possible to establish a thermal contact in the respective maintained measuring wheel position, in order, for instance, to maintain a desired temperature constant or to vary it in a defined manner.